Field of the Invention
The present disclosure relates to lithographic apparatus. The disclosure relates in particular to the measurement of local height deviations, which are important for focusing in optical lithography. The disclosure further relates to methods of manufacturing devices by lithography, and to data processing apparatuses and computer program products for implementing parts of such apparatus and methods.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The pattern is imaged onto the target portion of the substrate using a lenses (or mirrors) forming a projection system. When imaging the pattern onto the substrate it is desirable to ensure that an uppermost surface of the substrate (i.e. the surface onto which the pattern is to be imaged) lies within the focal plane of the projection system.
The surface of a substrate on which a pattern should be projected is never perfectly flat, but presents many local height deviations on both a large scale and smaller scale. Failure to adjust the focus of the projection system may result in poor patterning performance and consequently poor performance of the manufacturing process as a whole. Performance parameters such as critical dimension (CD) and CD uniformity in particular will be degraded by poor focus.
To measure these local height deviations, level sensors are normally integrated in the lithographic apparatus. These are optical sensors used to measure and the vertical position of the uppermost surface of the substrate at points all across the substrate, after it has been loaded into the lithographic apparatus. This set of measurements are stored in the form of a height map. The map is then used during exposure (patterning) to ensure that each portion of the substrate lies in the focal plane of the projection lens. Typically the height of a substrate table bearing the substrate will be adjusted continuously during exposure of successive portions on a substrate.
A known problem with optical level sensors is that different substrates, and different parts of a substrate, will interact differently with the measurement beams of radiation. In other words, the height measurements obtained by a level sensor are subject to process-dependent effects and do not always give the true height. In particular, an apparent surface depression is known to be caused when light reflected from the substrate is subject to so-called Goos-Haenchen shift. This effect is different for different materials and depends heavily on the structure and materials of several layers. Therefore the apparent surface depression can vary significantly from layer to layer, and between regions across the substrate. A heavily metallized region will reflect light more reliably than a region with predominantly dielectric material, for example. In U.S. Pat. No. 7,265,364 B2 (Tuenissen et al., ASML), a modified level sensor is described which measures height separately using S-polarized and P-polarized light to detect areas of high process dependency. The results of this detection are used to discard or correct height measurements obtained from particularly troublesome regions of the substrate.
US 2010/0233600 A1 (den Boef et al.) proposes an alternative level sensor using radiation in an ultraviolet (UV) wavelength range similar to that used in the projection system. The shorter wavelength radiation is less susceptible to process dependency. However, such UV level sensors are not available in existing apparatuses, and will still only measure the height at specific sample locations.
Another approach to correct for process dependency in leveling systems is to use ‘non optical’ inspection apparatuses to complement and calibrate the optical sensor. These non-optical sensors may be for example such as profilometers (available for example from KLA-Tencor, San Jose, Calif.) and/or Air Gauge sensors (as described for example by F. Kahlenberg et al., Proc. SPIE 6520, Optical Microlithography XX, 65200Z (Mar. 27, 2007)). These non-optical sensors can be employed to deliver true height measurements, and to calculate corrections for use with optical sensor height measurements. However, the use of these sensors may be highly time consuming and they are not generally suitable for integration in the lithographic apparatus itself. Therefore, profilometers and Air Gauge sensors may be employed, for instance, to measure the height of specific fields and/or specific areas of the field of a few wafer samples in an “off-line” situation, that is outside the mass production process. Height measurements obtained with Air Gauge sensors may then be used, for example, to obtain a map of corrections to be applied to measurements obtained with optical sensors.
It is also noted at this point that the spatial resolution of level sensors and non-optical inspection apparatuses may be limited. Existing sensors do not allow the height map to represent precisely the boundary between regions of very different process dependency. For example, level sensors may not be able to measure sharp area boundaries in product topography. An example of extreme topography is seen for example in the manufacture of 3D NAND devices, where on backend layers (layers formed at a late stage in the manufacturing process) substantial height deviations can be seen over a distance ˜100 μm.